Microchannels are being considered in many advanced heat transfer applications including automotive and stationary fuel cells as well as electronics cooling. However, there are a number of fundamental issues from the heat transfer and fluid mechanics perspectives that still remain unresolved. The present work focuses on obtaining the fundamental heat transfer data and two-phase flow patterns present during flow boiling in microchannels. An experimental investigation is performed for flow boiling using water in six parallel, horizontal microchannels with a hydraulic diameter of 207 μm. The ranges of parameters are: mass flux from 157 to 1782 kg/m^{2}s, heat flux from 5 to 930 kW/m^{2}, inlet temperature of 22°C, quality from sub-cooled to 1.0, and atmospheric pressure at the exit. The corresponding single-phase, all-liquid flow Reynolds number range at the saturation conditions is from 116 to 1318. The measured single-phase, adiabatic pressure drop agreed with the conventional theory within the experimental error. The experimental single-phase Nusselt number was found to be between the constant heat flux and the constant wall temperature boundary conditions, corresponding to $NuH$ and $NuT$ respectively. The flow visualization demonstrates that the flow reversal condition in parallel flow channels is due to bubble nucleation followed by its rapid growth. In addition, the dry-out condition is observed, showing a change in the contact angles of the liquid-vapor interface. The local flow boiling heat transfer coefficient exhibits a decreasing trend with increasing quality. A comparison with the nucleate boiling dominant regime of a flow boiling correlation shows good agreement, except for the large peak in two-phase heat transfer coefficient observed at the onset of nucleate boiling.

*Proceedings of International Mechanical Engineering Congress and Exposition*, ASME, New York.

*Proceedings of 1st International Conference on Minichannels and Microchannels*, ASME, New York, pp. 115–127.

*Canadian Journal of Chemical Engineering*,

**71**(5), pp. 657–666.

*International Mechanical Engineering Congress and Exposition*, ASME, New York.

*Heat and Technology*,

**14**(2), pp. 47–54.

*National Heat Transfer Conference*,

**7**, HTD-329, pp. 167–178.

*Proceedings of 33rd National Heat Transfer Conference*, ASME, New York.

*Proceedings of 6th UK Heat Transfer Conference*.

*Proceedings of the ASME Heat Transfer Division 4*, HTD-Vol. 366–4, pp. 55–63.

*Journal of Microelectromechanical Systems*,

**10**(1), pp. 80–87.

*35th Proceedings of National Heat Transfer Conference*, ASME, New York.

*4th Proceedings of International Conference on Multiphase Flow*, Efstathios E. Michaelides, ed.

*Proceedings of the ASME International Mechanical Engineering Congress and Exposition*, ASME, New York.

*Transactions of IChemE 79, Part A*, pp. 417–424.

*Symposium on Compact Heat Exchangers on the 60th Birthday of Ramesh K. Shah*, pp. 401–406.

*Journal of Microelectromechanical Systems*,

**11**(1), pp. 12–19.

^{2}Electronics Cooling,”

*Proceedings of ASME Summer Heat Transfer Conference*, ASME, New York.

*First International Conference on Microchannels and Minichannels*, ASME, New York.

*Proceedings of ASME Summer Heat Transfer Conference*, ASME, New York.

*Proceedings of ASME Summer Heat Transfer Conference*, ASME, New York.

*First International Conference on Microchannels and Minichannels*, ASME, New York.

*First International Conference on Microchannels and Minichannels*, ASME, New York.

*Advances in Heat Transfer*, Academic, New York.

*Proceedings of 1st International Conference on Minichannels and Microchannels*, ASME, New York, pp. 567–579.

*Proceedings of International Mechanical Engineering Congress and Exposition*, ASME, New York.

*Proceedings of First International Conference on Microchannels and Minichannels*, S. G. Kandlikar, ed., ASME, New York.